1. Field of the Invention
The invention relates to a process for the separation of plastic particles of a plastic mixture of plastics of a chemically different type which partly have an overlapping and partly a different density range, e.g. polyethylene (PE), polyethylene terephthalate (PET), polypropylene (PP), polystyrene (PS), and polyvinyl chloride (PVC).
2. The Prior Art
Such different types of plastic occur as waste, for example when different one-way bottles are mixed. For example, still waters are predominantly filled in 1.5-liter PVC-bottles, whereas other beverages are sold in so-called PET-bottles. In Western Europe alone, 1.4 billion PET-bottles are manufactured annually. The bottles have, as a rule, a polyethylene screw cap, whereby the PET-bottles may have a bottom part made of polyethylene as well. Direct recycling of the mixed bottle plastics is not possible because PET melts only at 260.degree. C., whereas PVC decomposes with separation of HCl already above the softening temperature of 160.degree. C. There are, therefore, no appreciable recycling possibilities, so that the waste plastics have not been collected heretofore but eliminated via the household refuse, i.e., they are finally incinerated or deposited.
Furthermore, it is not possible, as a rule, to achieve any profits for mixed PVC-containing plastics. The fact is rather that the reuser frequently demands a credit that is oriented on the dumping costs saved.
As opposed to the above, there is a market for purely sorted recycling plastics since long, whereby the prices are oriented on the prices for new material. Up to 60% of the new material is achieved for recycling material depending on the quality. Thus there is much interest in processes for the separation of mixed plastics.
The processes known from the state of the art for the separation of plastic particles of plastics of a chemically different type operate with plants separating according to the density, for example hydrocyclones. Said process, however, fails in connection with plastics that are in the same density range such as, for example, PET (density about 1.37 to 1.38 g/cm.sup.3) and PVC (density about 1.38 g/cm.sup.3). However, the separation of polyethylene (PE) from the other two plastic types PET and PVC is possible because of the different density of 0.95 g/cm.sup.3. The separation of plastics that are in the same density range can be carried out, for example electrostatically.
It is known from DE-PS 30 35 649 to separate plastics electrostatically in a free-fall separator.
However, it has been found that in the separation of a plastic mixture with three or four different types of plastics, thus for example PE, PET, PS and PVC with one of said processes, a large quantity of medium material is collected, or that the deposits on the respective electrode have only an insufficient degree of purity. Furthermore, the medium material has a high component of at least one of the plastics used.